Grain non-oriented electric steel strips are used mainly in the manufacturing of cores of rotary electrical machines.
The products available on the market are classified on the basis of the magnetic properties thereof (defined by standard UNI EN 10106).
Such magnetic characteristics are associated to a crystalline structure of the components characterised by mean dimensions of the crystalline grain typically not greater than 0.25 mm and with a crystallographic weave as isotropic as possible in the plane of the rolled product to guarantee a magnetic behaviour that is similar for the various angles of application of the magnetic field during operation of electrical machines.
The main qualifying magnetic characteristics are the magnetic losses measured at specific induction working conditions and magnetisation frequency and the polarisation levels attainable at specific values of magnetic field applied in accordance with the IEC standard 60404-2 and IEC 60404-3. The existing technologies for manufacturing grain non-oriented electric steel strip are many and exploit, for reduction of the magnetic losses, some strategies, such as the reduction of the final thickness of the rolled products and/or the increase in electrical resistivity of the metal alloy by addition of elements such as Si, Al, Mn . . . , etc.
A further metallurgic characteristic exploited for improving the magnetic quality of the products is the maximum containment of the second non-metallic phases present in the metallic matrix of the steel, such as for example sulphides, nitrides, carbides and oxides. For this purpose the most modern production practices include manufacturing of steels having very low content of sulphur, nitrogen, carbon and oxygen already in the stage of solidification of the molten steel during slab casting.
A further controlled physical characteristic in rolled products is the surface quality of the sheets. In particular the best products are characterised by having surfaces having a low degree of roughness and free (or nearly free) from thermal oxidation.
Grain non-oriented electric steel strips, typically made of a Fe—Si alloy with a silicon content variable between 0.1% and 3.3% by weight, are manufactured starting from a molten alloy and solidified into slabs; the slabs are hot-rolled to obtain sheets of rolled product which are then subjected to cold-rolling up to the final application thickness.
With the exception of direct Strip-casting solidification, described for some time in the scientific and international patent literature but which does not at present include any reference industrial application, the slabs for manufacture of grain non-oriented strip are produced by continuous solidification at variable thicknesses, according to the technology used, in a range comprised between 20 mm and 300 mm.
Hot-rolling of the slabs is carried out in sometimes very different ways according to the technology and plant used but in any case in a temperature range comprised between 1300° C. and 700° C., for obtaining hot-rolled products having thicknesses variable between 2.5 mm and 1 mm.
Products for final use, typically offered on the market, are of a thickness that is variable between 1 mm and 0.35 mm for conventional applications and of lower thickness up to 0.2 mm for special and high frequency applications.
All the existing production processes are characterised by a reduction by hot-rolling (typically at temperatures >700° C.) to an amount comprised between 90-95% (thin-slab technology) and 98-99% (thick-slab technology) and by a reduction by cold-rolling (typically at temperatures <300° C.) to an amount generally comprised between 60% and 80%.
Then, in particular, in the most modern technologies, there is the tendency to produce hot-rolled sheets that are ever-thinner for reducing as far as possible the amount of cold-rolling applied. The claimed advantages are a reduction in cost and improvement of performance connected to a cold-rolling that is less expensive and at the same time an improvement of the magnetic quality associated to an improvement in the crystallographic weaves obtainable in the finished products.
U.S. Pat. No. 676,412 discloses a manufacturing method of grain non-oriented magnetic strip starting from hot-rolled sheets having a thickness of less than 1.5 mm. The method described in this document includes a silicon and aluminium content that is limited ([% S]+2[% Al]≤1.8) and even if it includes the possibility of adding further alloying elements such as P, Sn, Sb, Zr, V, Ti, N, Ni, Co, Nb, B up to an overall value of not greater than 1.5% the magnetic losses obtainable are therefore of poor quality with respect to the range of high-quality products required by today's market.
US 2005/0067053 A1 describes a process for hot-rolling manufacture of strip suitable for manufacture of grain non-oriented magnetic strip with a maximum hot-rolled strip thickness of 1.8 mm or 1.2 mm. The content of Si and Al that can be adopted is in this case higher but in any case limited to [% Si]+2[% Al]≤5%. In the document describing a specific crystalline structuring of the matrix of the hot-rolled product, there are no specific advantages in terms of high magnetic characteristics of the products that can be realised with such process.
The magnetic quality of the grain non-oriented strips, especially in relation to the use destination in electrical machines working at high frequency, is mainly regulated by the electrical resistivity of the alloy (content of elements such as Si, Al, Mn, . . . ), by the thickness of the strip and the polarisability of the material in the rotation plane of the magnetic field. Further, the mean size of the crystalline grains of the products must be regulated within specific very narrow dimensional ranges and optimised for the working frequency of the electrical machines to which they are destined.
In the industrial manufacturing of these products, however, there are important limitations to exploitation of said metallurgic levers for control of the quality, mainly in connection with practical factors, in particular:                an increase in the alloying elements increasing the electrical resistivity of the metal alloy leads to an increase in mechanical fragility up to critical levels wherein the reduction of the physical manufacturing performance makes the production processes no longer advantageous;        cold rolling to very slim thicknesses incurs high costs, especially when associated to the problems of fragility characteristic of the alloys under discussion;        it is difficult to guarantee control of the product microstructures (in terms of distribution of the grain size and crystalline structure) by which the characteristics of magnetic loss and magnetic polarisation are critically influenced, due to the high susceptibility of the microstructure to even small fluctuations in chemical composition of the alloys and the thermo-mechanical treatment conditions and final annealing.        